1. Field of the Invention
The present invention relates to an electromagnetic coupling for operating a main clutch into ON (connecting) and OFF (disconnecting) states by means of a pilot clutch operated by electromagnetic force.
2. Description of the Prior Art
A conventional electromagnetic coupling of this type is made up, for example, as shown in FIG. 1. Referring to the Figure, the electromagnetic coupling includes an input shaft 1 that is connected to an engine (not shown) side, an output shaft 2 for transmitting power to rear wheels, and a main clutch 3 for connecting and disconnecting the input shaft 1 to and from the output shaft 2. The main clutch 3 comprises inner plates 3a whose inner diameter sides are coupled in spline engagement with the outer periphery of the input shaft 1 and outer plates 3b whose outer diameter sides are coupled in spline engagement with the inner periphery of a casing 2a united to the output shaft 2 in a manner to be described later, the inner and outer plates 3a and 3b being alternately positioned. A fixed plate 4 is fastened to an end of the casing 21 and a thrust means 5 has a push plate 6 such that pushing the inner and outer plates 3a and 3b against the fixed plate 4 with the push plate 6 under a pressure engages the input shaft 1 with the output shaft 2 and removing the pushing pressure or thrust disengages the output shaft 2 from the input shaft 1. See, e. g., JP 2002-266904 A (pages 5 to 6 and FIG. 1).
The thrust means 5 comprises the abovementioned push plate 6, a backing plate 7 disposed behind the push plate 6, and a thrust generating mechanism 8 for actuating the push plate 6 to push with respect to the backing plate 7 via balls disposed between them.
The inner diameter side of the push plate 6 is coupled in spline engagement with the input shaft 1. And, the backing plate 7 with its end face supported by a thrust bearing 16 is supported rotatably relative to a boss section 2b of the output shaft 2 and has its inner diameter side fitted with the outer periphery of the input shaft 1 so that it can be rotated while in sliding contact with the latter. Further, this backing plate 7 is coupled via a pilot clutch 9 lying outwards thereof to the casing 2a united with the output shaft 2 so that it can engage with and disengage from the casing 2a in a direction in which it is rotated.
In the pilot clutch 9, a retaining ring 10 is axially inserted into an annular space formed between the outer periphery of the backing plate 7 and an inner periphery of the casing 2a so that it can be axially moved in sliding contact with these peripheries in that space to an extent that it comes into contact with a stop ring 17. And, between the retaining ring 10 and a rotor section 2a′ of the casing 2a there are positioned alternately clutch plates 9a of a first group whose inner diameter sides are coupled in spline engagement with the outer periphery of the backing plate 7 and clutch plates 9b of a second group whose outer diameter sides are coupled in spline engagement with the inner periphery of the casing 2a. And, axially rearwards (at the left hand side as shown) of the rotor section 2a′ of the casing 2a there is provided a ring shaped electromagnetic coil 11 for electromagnetically attracting the retaining ring 10 towards the rotor section 2a′ of the casing 2a to bring the clutch plates 9a, 9b into pressure contact with one another, thereby turning the pilot clutch 9 into its ON state.
The electromagnetic coil 11 is disposed in a ring shaped chamber 12 formed by a stepped recess formed in the casing 2a and an outer periphery of the boss section 2b of the output shaft 2 and arranged therein so that it is juxtaposed with the pilot clutch 9 across the rotor section 2a′. And, this electromagnetic coil 11 is supported from a cover member 13 which is securely connected to a frame (not shown) and which is fitted over and rotatably coupled to the output shaft 2 via a bearing 18.
Opposed to the electromagnetic coil 11, the rotor section 2a′ of the casing 2a and the flange 2b′ of the boss section 2b, which are positioned diametrically outwards and inwards, respectively, as separate members, are united together with a ring of nonmagnetic material 14 that is composed of stainless steel as a nonmagnetic material. Also, the clutch plates 9a, 9b of the pilot clutch 9 are provided at respective diametrical positions thereof flush with the inner and outer peripheral surfaces of the ring of nonmagnetic material 14 with cutouts 19 and bridges (not indicated) interconnecting them for preventing a magnetic force on a magnetic loop 15 of the electromagnetic coil 11 from short-circuiting.
The electromagnetic coupling constructed as mentioned above operates as stated below.
Turning current conduction to the electromagnetic coil 11 ON generates the magnetic loop 15 in which the retaining ring 10 is attracted magnetically by the electromagnetic coil 11. This causes the clutch plates 9a and 9b in the pilot clutch 9 to be pushed against the rotor section 2a′ of the casing 2a whereby the pilot clutch 9 is turned into its ON state and the backing plate 7 in the thrust means 5 is then brought into engagement with the casing 2a, namely with the output shaft 2. In a 4WD coupling, its 4 WD state is taken by this state.
If in this state a difference in speed of rotation between the input and output shafts 1 and 2 is produced due to a slip of the rear wheels or the like, then a corresponding difference in speed of rotation is produced between the backing plate 7 in engagement with the output shaft 2 and the push plate 6 in engagement with the input shaft 1. Then, the thrust generating mechanism 8 is brought into operation by such a difference in speed of rotation to push the push plate 6 with respect to the backing plate 7, thereby turning the main clutch 3 into its ON state. Thus, the input and output shafts 1 and 2 are interconnected and rotated in one united body.
The abovementioned thrust generating mechanism 8 comprises balls 8a each of which as shown in FIG. 2A is rollably interposed between a pair of opposed reentrant cam surfaces 6a and 7a of the push and backing plates 6 and 7. When a difference in speed of rotation is produced between the push and backing plates 6 and 7, the ball 8a tends to run on the reentrant cam surfaces 6a and 7a as shown in FIG. 2B with the result that the push plate 6 is moved away from the backing plate 7 in the direction of the arrow shown. Consequently, the plates 3a, 3b of the main clutch 3 is moved and pushed to the fixed plate 4.
The pilot clutch in a conventional electromagnetic coupling as mentioned above must have its capacity increased to meet with the need for a greater torque transmission of the main clutch. If a large torque is to be transmitted in the main clutch, an insufficient capacity of the pilot clutch creates troubles such as that the clutch slips. Also, in strengthening the magnetic force, measures have hitherto be taken of devising the configurations and thicknesses of inner and outer diametrical parts of the housing in which the electromagnetic coil is accommodated. As a result, a considerable number of man-hour and its attendant time period have so far been required in manufacturing an electromagnetic coupling.
On the other hand, while increasing the magnitude of electric current to the electromagnetic coil may strengthening the magnetic field it produces to increase the electromagnetic force, the magnetic saturation of a magnetic material imposes a limitation on increasing the electromagnetic force by increasing the current magnitude.